Stillwell observes the yard from above a man-made hill - one of two at Bailey Yard - that make a big difference on the flat plains west of North Platte. The humps, as they are known, replace the work a locomotive would otherwise do.

"Today, we still have rail yards that are flat. In other words, they're on a flat piece of ground," said Union Pacific's Mark Davis. "What you'll do in that case to sort cars is the locomotive will shove or push the rail cars, and then once they're uncoupled, you stop that forward movement, the car separates and free-wheels then into the rail yard."

"A hump yard is a little more efficient in sorting the rail cars, and it takes less time to do that, so you're looking at (more) efficiency," he continued. "For us, every minute counts when you're moving goods across country, so the hump yard helps you achieve a little bit more efficient sorting of rail cars that need to be sorted."

As a pin puller walks alongside the slow-moving cars inching up the hill, he separates them near the crest, sending them downhill. Sensors along the tracks monitor how fast each car is moving. If it's going too fast, it could be damaged when coupling with the car in front of it.

Sounds simple enough, but the hump is really the start of a sophisticated system that considers car weight, distance the car must travel, wind speed and direction and even air temperature.

Greg Martin, Union Pacific's director of transportation systems development, said all these things impact how fast the car travels.

"That car is being classified and getting sorted, so it can make its outbound train destination, and the system is telling it how to get there safely."

To keep cars from moving too fast, a series of brakes, called retarders, are told by a computer how much to slow each car down.

Sounding something like the air brakes on a roller coaster, the retarders are located between the hump and the bowl, where the car will meet its new train.

"What the process control system does is it says, in order for me to couple way down that bowl track, a thousand feet down that bowl track, I need an exit velocity from this group retarder of 13 miles an hour," Martin explained, giving an example situation. "So if it enters that group retarder at 15 miles an hour, it's going to try to dissipate enough energy to meet that objective."

"On every car it's different," he added. "So, you get a loaded car, you don't want as high a velocity, right? So your exit velocity will be a lot lower than a empty car, which needs more energy to get down there."

The whole process is based on a fundamental theory of physics known as the conservation of energy. The theory says energy is never created or destroyed. Energy at the crest of the hump is equivalent to the same energy at the bottom, Gravity transfers that energy from potential to kinetic.

"So the system in this case is the hump yard," Martin said. "And it starts with, let's just, for example, say it's a free rolling car, and it has minimal kinetic energy at the top and a lot of potential energy at the top of the hill. So it's moving a little bit. That's where it's getting its kinetic energy. "

Rolling resistance results in a transfer of energy. The resistance can be the pressure applied by the retarders, friction from the wheels on the rails or from a head wind.

"The energy at the top of the hill, which is the initial energy potential plus kinetic at the top of the hill, is equal to the potential and kinetic energy at the bottom of the hill," he said. "Hump yards were designed many, many years ago. And basically, it's taken advantage of energy that from gravity. Because it's free. So if you have a free-rolling car, going down that hill, you know, then that's economically positive."

Scenes of a fast-moving train are common. At Bailey Yard, Mark Davis says a much slower speed is important.

"Something above four miles an hour, things will start shifting in a railcar if they're not tied down properly when they're loaded, so in order to avoid damage to the commodities, four miles an hour or less, that's the optimum."

As Stillwell sits perched in the tower above the hump, he makes sure the tracks are clear before another car heads down the hill.

"So say the job is to get the cars from origin, be it at Denver, Salt Lake, Seattle, what have you, into North Platte, and once it gets to North Platte, all these cars are going to all parts of the United States and the world."

UP's Bailey Yard is considered the largest classification yard in the world. It has 315 miles of track, is eight miles long and two miles wide.